Contributing guide update

[eliotwrobson]

The contributing guide is pretty out of date (doesn't include discussion of black or isort for style, for instance), and doesn't include instructions on getting documentation working locally. The documentation site itself also doesn't link to the contributing.md file.

This is probably going to get brought up in the upcoming review, so opening this issue in advance of that so there's not as much of a rush to dig through old issues. Cc @caleb531

Docs local dev instructions: #160 (comment)

[eliotwrobson]

Changes merged in #210